An opposition lawmaker accused the government of organizing that attack and the police of not protecting the protesters. He said about 50 people may have been injured.
Opposition lawmakers and their supporters were heading toward a prison in the capital of Colombo when they came under attack. Photographers covering the protest were beaten and one of their cameras was taken away. It was not possible to immediately verify the number of injured.
Sri Lanka celebrated the 63rd anniversary of its independence from Britain on Friday, but opposition lawmaker Tissa Attanayake said the protest was organized to highlight the lack of freedom that persists in the country. They were also demanding the release of former army commander Sarath Fonseka, who has become the leader of the opposition.
"An organized mob attacked us. It was very clear that the government was behind it and the police were just inactive," Attanayake said.
Police spokesman Prishantha Jayakody said he was unaware of the incident. Government ministers could not be immediately reached for comment.
Fonseka is serving a 30-month prison term after being convicted of corruption. His supporters allege he is being punished by the government for daring to challenge President Mahinda Rajapaksa in last year's election.
Rajapaksa and Fonseka were allies in the country's civil war and were hailed heroes by many Sri Lankans for defeating the separatist Tamil Tiger rebels in 2009, ending a quarter-century civil war.
They fell out soon after, though, and Fonseka resigned from the army to contest the January 2010 presidential election.
He was arrested weeks after his defeat on allegations he was plotting a coup but was later charged in a military court with corruption and planning his political career while still in the army.